Automobile engine exhaust systems have traditionally relied on pipes made completely of metal to provide passage of the spent exhaust gases from the engine compartment to the rear of the vehicle, where the gases are released to the atmosphere. Although this method of exhausting gases is effective, considerable thermal energy is given off by conduction through the metal exhaust pipe wall, resulting in the need to protect sensitive components near the pipe from the connected and radiated heat. In addition, conventional metal exhaust pipes are subject to corrosion due to chemical and physical attack in the use environment.
The fabrication of a composite pipe with a layer of insulating material to reduce thermal energy transfer between a thin-walled inner metal tube and an outer non-metal casing would alleviate, or even eliminate, the above-mentioned disadvantages. It would reduce the need for vehicle heat shields and allow design flexibility with respect to heat sensitive materials and components located near the pipe. The thermal load on a vehicle's air conditioning unit would be lessened by reducing the underbody heat. Additionally, since the composite pipe would transfer less heat, it would desirably provide the heat sooner to the catalyst in the catalytic converter causing faster light off, thus improving catalytic efficiency. By maintaining the exhaust gas temperature and resultant gas velocity, a scavenging action of the cylinders is improved, which in turn contributes to overall engine efficiency. The composite pipe also would have extended durability and .improved NVH characteristics as well as the advantage of possible weight reduction.